In recent years, advances in technology, as well as ever evolving tastes in style, have led to substantial changes in the design of automobiles. One of the changes involves the power usage and complexity of the various electrical systems within automobiles, particularly alternative fuel vehicles, such as hybrid, electric, and fuel cell vehicles.
In most hybrid vehicles, energy storage devices, such as capacitors, are often used to improve efficiency by capturing energy within the powertrain system or supplying additional power during periods of operation when a primary energy source cannot supply the required power quickly enough. For example, regenerative braking may be used to capture energy by converting kinetic energy to electrical energy and storing the electrical energy in a bank of capacitors for later use. In order to accommodate high-voltage operation within automobiles, capacitor banks or supercapacitors are often used because they have the ability to quickly store energy and can be discharged at a much higher rate than other energy sources.
However, capacitors may retain a charge long after power is removed from a circuit or an automobile is turned off. Therefore, high-voltage capacitors should be properly discharged after turning off a vehicle or before accessing the equipment housing the capacitors. Discharging a capacitor is typically accomplished by placing a discharge or bleed resistor across the capacitor or bus terminals in parallel.
In an automobile, there are possible fault conditions that may result in a constant voltage across the capacitor terminals. If a fault is not properly detected and protected against, attempting to discharge the capacitor will overheat and destroy the discharge resistor. Moreover, the failure of a discharge resistor prevents future discharge, resulting in voltage being retained on a capacitor or within the circuit for an extended period of time.
Previous discharge circuits and methods require unacceptably high average power dissipation in a discharge resistor during fault conditions and are not easily adaptable to higher voltage levels. These designs require discharge resistors with the ability to handle high average power dissipation. These resistors generally occupy a larger surface area and often require additional harnesses, connectors, and heat sinks, which prevent the discharge resistors from being built on a circuit board. In addition to the spatial requirements, these discharge circuits are adapted more for the less frequent fault mode, rather than a normal operating mode.